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Unfortunately it sounds like the logic in your display board is just not working properly. Since the microwave does work on the normal time setting, you know the start button works. Try to unplug the microwave for 5 minutes to see if the electronics will reset. If it doesn't then the display board would need to be replaced to be able to change the power level.
Hello again. I'm just following up because I haven't heard back from you and I see this question is still open. Is there anything else I can help you with?